1. Field of the Invention
The present invention relates to an apparatus for the ejection of water from a running felt loop in a paper machine.
2. Description of the Related Art
In traditional devices, water is sucked out of the felt by the application of vacuum to one side of the felt and the purging action of a stream of air entering the other side of the felt. The water is thus conveyed into stationary suction slots or suction holes of a rotating suction roll.
According to another method, the water can be squeezed out by compressing the felt in a press nip. A so-called wringer press serving this purpose is very expensive, consumes a considerable amount of energy, and reduces the service life of the felt. For these reasons, wringer presses are rarely found in today's paper machines.
With the introduction of synthetic needle felts, through-purging with air has become the common method of felt dewatering in all cases where the water cannot be removed directly in the press nip. In this latter case, however, the press rolls must be fitted with special devices for holding the water at the nip exit, making the press rolls expensive and complex.
These rolls supporting the felt can be designed as suction rolls with a perforated rotating shell, or as grooved rolls with grooves of 2.5 to 5 mm depth and 0.5 to 1 mm width on their outer periphery. In the case that the press roll supporting the felt is a shoe press roll, no suction can be applied because the rotating flexible press sleeve must be unperforated in order to form a lubricant film on the pressing shoe.
Because of the special properties of its material, the press sleeve cannot be made very thick or hard. Therefore, only relatively shallow grooves of maximally 1.5 mm depth or blind drilled holes of 2 mm depth can be incorporated into the outer surface. This results in a limited water storage and removal capacity of these press sleeves that cannot be extended to meet the demands of many pressing applications. Therefore, some water must still be removed from the felt by air purging, demanding high permeability felts. Felts of higher permeability, however, include coarse fibers that mark or "emboss" the surface of the paper in a detrimental way and lead to an increased water reabsorption at the press nip exit back into the paper. If, on the other hand, the felt is made sufficiently fine-pored in order to produce an optimally smooth paper surface and to avoid water reabsorption, the concept of air purging will fail.